A tremendous amount of cellular metabolism is dedicated to producing ribosomes, the large ribonucleoprotein factories of protein synthesis responsible for translating mRNAs into proteins. Ribosomes are synthesized in the nucleolus following a series of endonucleolytic and exonucleolytic reactions that process the pre-rRNA to the 18S, 5.8S and 25S rRNAs. My laboratory has recently purified and characterized a new large RNP, the SSU (small subunit) processome, which is required for pre-18S rRNA processing. While our work has uncovered 30+ of the SSU processome proteins, we hypothesize that there may be other important components, including RNA helicases and endonucleases, that function with the SSU processome to mature the 18S rRNA. In addition, very little is known about the assembly of the SSU processome on the pre-rRNA and how it is related to pre-rRNA transcription. Our long-term goal is to understand the pre-rRNA processing, RNA folding and ribosome assembly steps essential to ribosome biogenesis. The objective of this application is to uncover different aspects, including the dynamic nature, of small ribosomal subunit (SSU) biogenesis. The central hypothesis is that formation of the SSU processome on the pre-188 rRNA is a critical part of SSU biogenesis. Our work is significant because the SSU processome is vital to gene expression, and therefore cell growth, in all eukaryotic cells. In Specific Aim 1, we will test the hypothesis that mutations in conserved motifs in DExD/H box RNA helicases can be used to more precisely define their function in SSU biogenesis. In Aim 2, we will further investigate the link between pre-rRNA transcription and processing mediated by the SSU processome components. In Aim 3, we will test the hypothesis that the proteins encoded by ORFS YOR004w and YDR339c encode endonucleases required for pre-18S rRNA processing. In Aim 4, we will test the hypothesis that there are other non-essential genes encoding proteins involved in pre-18S rRNA ribosome biogenesis.